Absolute Value Output Circuit Capable of Rectifying Micro Voltage

Absolute value output circuit capable of rectifying micro voltage

Function of circuit

Regardless of the polarity of the input signal, this circuit only outputs its amplitude with a single polarity, which can also be called a full wave rectifier circuit. Absolute value circuit is an application example of ideal diode circuit. It can accurately rectify small voltage, so it can be applied in rectifier circuit with large dynamic range. Its disadvantage is that the higher the frequency, the worse the accuracy by using the feedback of OP amplifier.

Circuit working principle

OP amplifier A1 acts as an ideal diode circuit with negative output. A2 is an adder with a ratio of 2:1 (R2 = 2R4). When the positive half cycle input, A1 output is the output after the inverted ei1 and B1 are inversely added. EO = - (e1-2ei) = EIO because R3: R4 = 2:1, - 2ei can be regarded as an equivalent input voltage of 2 times. When the negative half cycle input, A1 output is disconnected, EO = - (- E1 0) = EI, The input signal is passed as is.

This circuit uses feedback and is limited by the open-loop characteristics of OP amplifier. If it is used in high-frequency circuit, OP amplifier with large gain at high frequency must be selected.

R1 and R3 determine the resistance value of the input resistance. For low frequency, the resistance value of R1 and R3 can be 150k. The ratio of R1 and R2 in this circuit is only 1:1, not necessarily 15K. The ratio of R3 to R4 is 2:1. The resistance of E24 series can only be matched with 15K and 7.5K. If you want to select other resistance values, R4 can be connected in parallel with two resistors with the same resistance value as R3.

R5 is used to determine the magnification, usually a = R5 / R3 = 1, but a ≥ 1 can also be made.

Capacitor C1 plays the role of smoothing capacitor during full wave rectification. To smooth DC, its capacity should adapt to the signal frequency. If the full wave rectification is smooth, the DC output is: 2|ei|x, i.e. 0.6361|ei|. If the capacitance is increased, the ripple after rectification can be reduced, but the response will also be slow. The capacitance shall be determined according to the use requirements.

Selection of components

In this circuit, the relative accuracy of the resistance value is very important. Usually, the metal film resistance with an error of plus or minus 1% is selected. If the accuracy requirement is very high, the combined resistance IC with good relative accuracy can be selected. Two resistors with the same resistance value as R4 are also required. Only six elements are enough for the whole circuit.

It is easy to smooth the absolute value output into DC. However, if the frequency is high and there are requirements for the output waveform, it is very important to choose what kind of OP amplifier. The signals are added at R3 and R4. Since the signal flowing through R4 passes through the primary OP amplifier A1, there will be a time delay and the current waveform cannot be superimposed well. If the OP amplifier A1 adopts the high-speed type, the delay time can be shortened. However, as shown in figure a, on the path where the R3 input signal is directly superimposed, R3 uses two resistors of 7.5K each in series, and the middle is grounded through capacitor C1, so that several time delays of the signal can also be corrected.

instructions

There is no offset adjustment circuit in this circuit, so the level of input signal should be increased as much as possible. In order to improve the accuracy of low level, the offset of each OP amplifier should be eliminated. In order to eliminate the influence of DC offset of the previous stage, a DC isolation capacitor can be connected in series at the input end.

Address: http://www.elecfans.com/arTIcle/analog/2010/20100511217318.html

Absolute Value Output Circuit Capable of Rectifying Micro Voltage 1

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